The processing of raw primary sewage, especially in municipal type sewage plants results in two primary streams, namely a raw primary sewage sludge and a primary clarifier liquor. Further processing is dependent upon two general variables: the requirements of local environmental standards and the degree of dewatering required for the proposed method of sludge disposal.
Raw primary sewage sludge may be laden with pathogenic organisms and can be dewatered mechanically to as high as 50% solids. Such organisms are normally destroyed through anaerobic digestion. Digester stability is quite sensitive and difficult to control, stray chemicals or toxins may kill the operation. Also, due to a minor cellular structure the digested sludge becomes more difficult to dewater.
The primary clarifier supernatent liquor is rarely returned to the environment due to high biochemical oxygen demand (BOD) in the form of dissolved solids. Charcoal filters may be used to absorb these solids but this is rarely economical. Chemical treatment is expensive and inefficient while producing an additional, rather inert sludge. Aeration is commonly used to reduce BOD and an activated sludge is produced which is most difficult to dewater by any method due to a highly cellular structure.
In typical secondary treatment plants digested and activated sludges are mixed and the resulting "mixed digested sludge" may be dewatered on drying beds to about 20% solids.
The major objectives insofar as environmental considerations are concerned are the destruction of pathogenic organisms and viruses, and the removal of BOD that might damage recipient water quality.
The considerations that one must keep in mind in dewatering the sludge are whether the dewatered sludge is to be used in land spreading such as in reclamation of farm lands and strip mines; whether it is to be used as secure land fill; whether it is to be incinerated; or whether it is to be dumped in the ocean.
Secure landfill is presently the most common method of sludge disposal in the United States. Dewatered sludge of all types may be disposed of in this way. When landfill is available, emphasis may be placed entirely on the environmental considerations of sewage treatment.
Incineration requires a maximum of dewatering. Primary digested sludge dewatered on drying beds to 25-35% solids is well suited for incineration. Most large cities have not converted to secondary treatment plants nor do they have available land or sea for alternative disposal. Over 25% of the sludge incinerated in the U.S. is primary digested sludge. Mixed digested sludge is often incinerated due to a lack of alternatives. Higher costs are incurred due to an increased excess air requirement, increased moisture content, larger equipment and lower efficiency.
Ocean dumping, where practiced, also requires dewatering to reduce the costs of transportation. The infamous New York sludge situation is a case in which dewatered raw primary sewage sludge has caused extensive environmental damage. Mixed digested sludge would be the best for disposal in a marine environment. There should exist strong currents to disperse the solids.
The Environmental Protection Agency (EPA) has declared formal guidelines for sludge disposal and utilization. This includes terminating ocean dumping programs by 1981. The guidelines will also result in increased cost of secure landfill. The present EPA attitude apparently indicates a preference for land application. Land used for spreading and composting sludge, storage of compost residues and other applications has been redefined as an integral part of the treatment process and is now eligible for Federal funds in the form of 75% matching grants. EPA regulations outline the extent to which sewage quality must be protected from toxins by industrial pretreatment in maintaining the option of land application where possible.
Public land applications require digested or stabilized sludge with high water content for pumping and large areas of available land which cannot be used for human consumption crops. Municipal incineration systems require a minimum of land and can be engineered to dispose of all types of municipal and industrial sludge, but investment costs are high and consumption of valuable fossil fuels may become prohibitive. The drying system of this invention requires no land, a minimum investment and no supplemental fuels when using dewatered sludge. Actual costs will be lower than most public and municipal systems.